Electrostatic generator



Oct. 28, 1958 Filed July 2, 1957 L. E. wlLsoN, JR 2,858,460

ELECTROSTATIC GENERATOR 2 Sheets-Sheet 1 Oct. 28, 1958 w so JR 2,858,460

ELECTROSTATIC GENERATOR Fil e d July 2, 1957 2 Sheets-Sheet 2 tates ELECTRDSTATIC GENERATOR Application July 2, 1957, Serial No. 669,619

Claims. (Cl. 31(l-5) This invention relates to electrostatic generators and in particular to an improved column construction for belttype electrostatic generators.

Electrostatic generators are well known and are disclosed, for example, in United States Patent No. 1,991,236 to Van de Graaff and No. 2,252,668 to Trump and at vol. XI, page 1 of Reports on Progress in Physics (1948). These devices generate high voltage by carrying electric charge to a hollow electrode on an insulating belt. The belt is generally supported on two pulleys, one of which lies within the hollow electrode and the other of which is at ground potential. Since the hollow electrode and the pulley within it are at high voltage, the mechanical support for them, generally referred to as the column, must have insulating properties as well as the proper rigidity to facilitate tracking of the belt on the pulleys. The optimum column construction is therefore necessarily a compromise between the ideal column from an electrostatic point of view and the ideal column from a mechanical point of view. The mechanical problems associated with the column become particularly acute in horizontal generators. Such horizontal mounting is frequently required in connection with electrostatic particle accelerators, either for reasons of space or because the accelerated particles must be injected into an orbital machine whose orbit lies in a horizontal plane.

The invention comprehends a novel column construction for electrostatic generators wherein both electrostatic and mechanical properties are maximized without compromising either property for the sake of the other.

Since the difficulties are greatest in a horizontally mounted machine, the invention will be described with reference to such a device, but it is to be understood that the invention is not limited thereto but includes vertical columns as well. In a horizontally mounted column, there are two major problems, one being the stresses in the column structure, and the other being the displacement of the high voltage terminal due to the bending of the column. in general, compactness requires that the column, and the acceleration tube be arranged close to each other; and so, if the acceleration tube is horizontally mounted, the column must be mounted likewise. Electrostatic generators are usually enclosed within a tank containing insulating gas under pressure which can insulate a given voltage across a smaller gap than can a solid insulator, owing to the occurrence of breakdown at the surface of a solid insulator. Therefore, the length of the acceleration tube and of the column are each greater than the minimum distance between the high voltage terminal and the tank which the insulating gas can insulate; and, accordingly, if the acceleration tube is horizontally mounted, the volume of the tank is minimized by arranging the column horizontally, even though a vertical column would provide optimum mechanical support. Moreover, electrostatic requirements virtually demand that the column be mounted next to the accelice 2 eration tube. For this reason the column is mounted in cantilever horizontally.

As set forth in the above-cited publications, the electrostatic properties of the column are improved by subdividing the total voltage by means of a series of equipotential planes; that is to say, the column comprises a series of flat metal sheets separated by insulating members which are cemented to the metal sheets. In accordance with the invention, the equipotential planes are used to assist in reducing the stresses in the glass and in reducing the vertical displacement of the high voltage terminal. Both displacement and stresses are also reduced by the provision of a heavy spring plane at the base of the column, and displacement is further reduced without appreciably increasing stress by stiffening the equipotential planes by means of rods which serve to increase the moment of inertia of said planes, thus restraining fiat-plate bending of these planes. These rods also serve as supports for belt spacers and gradient bars. In accordance with the invention, the cross section of the column is a narrow rectangle whose length is arranged vertically. The shape of the column is not only adapted to maximum mechanical support but is also sufficiently compact and of proper shape to fit within the runs of the belt, and the pulleys which support the belt are accordingly arranged with their axes vertical. This enables a much more eflicient utilization of space inside the column, whose lateral boundary facing the tank must be generally cylindrical, and the space within the runs of the belt, which is usually wasted in the conventional electrostatic generator, is used efficiently.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawings in which:

Fig. 1 is a somewhat diagrammatic side view, partly broken away, of an electrostatic generator embodying the invention;

Fig. 2 is a somewhat diagrammatic top view, partly broken. away, of the electrostatic generator shown in Fig. 1;

Fig. 3 is a side view, partly in section of the column of the electrostatic generator shown in Fig. 1;

Fig. 4 is a section taken along the line 4-4 of Fig. 3;

Fig. 5 is a section taken along the line 55 of Fig. 4;

Fig. 6 is a simplified diagram showing an electrostatic generator which is horizontally supplied in cantilever and of which the column comprises essentially a single beam rigidly fixed to the base plate;

Fig. 7 is a moment diagram showing the distribution of the bending moments in the column of Fig. 6;

Fig. 8 is a diagram similar to that of Fig. 6 but wherein the column comprises essentially two beams in vertical alignment supported by pins and wherein a series of equipotential planes are provided in accordance with the invention;

Fig. 9 is a moment diagram similar to that of Fig. 7 and showing the distribution of the bending moments of the column of Fig. 8.

Fig. 10 is a diagram similar to that of Fig. 8 wherein a heavy spring plane is provided in accordance with the invention;

Fig. 11 is a moment diagram similar to that of Fig. 9 and showing the distribution of the bending moments in the column of Fig. 10; and

Fig. 12 is a stress diagram illustrating the distribution of the total stresses in the column of Fig. 10.

Referring to the drawings, and first to Figs. 1 and 2 thereof, a column 1 constructed in accordance with the invention is supported in cantilever by two pins 2 at the grounded end thereof. The two pins 2 engage two clcvis blocks 3 at the base of the column 1 and. also correspondin apertures in a support frame 4. The

grounded pulley 5, which is an inverted motor, is supported upon the support frame 4 which is in turn connected by means of other pins 6 to a pair of stanchions 7 mounted on a base plate 8. The high voltage terminal assembly 9, including the upper pulley 10, which is an inverted alternator, is supported in a similar manner upon clevis blocks 11 at the terminal end of the column 1. The entire device is enclosed within a tank 12 which is bolted to the base plate 8 filled with insulating gas under pressure. An endless belt 13 travels around the pulleys 5,10, and around the column 1 upon which are supported a series of toroidal equipotential rings 14 which provide electrostatic shielding of the belt 13 and other portions of the apparatus as well as an electrostatic geometry which is compatible with the cylindrical pressure vessel or tank 12 in which the unit is housed.

Referring now to Figs. 3, 4 and 5, the column 1 itself comprises a multiplicity of alternating column plane assemblies 15 and pairs of glass column insulators 16. Each column plane assembly 15 has a generally rectangular configuration with its longer dimension vertically disposed in a horizontal machine. Each pair of adjacent column plane assemblies 15 are jointed together by being cemented to a pair of glass column insulators 16 of generally rectangular shape. Each column plane assembly 15 includes an equipotential plane 17 comprising a rectangular web or plane of stainless steel which may be for example of an inch in thickness, 2 feet long and inches wide. The equipotential plane 17 is sand blasted to a smooth finish on both faces at the area to which the glass column insulator 16 is to be cemented. A belt spacer support 18 is press fitted over each long edge of the equipotential plane 17 in order to increase the moment of inertia of the plane and thus reduce displacement of the high voltage terminal 9. In order to accommodate surge discharges and protect the glass column insulators 16, spark gaps are provided. These spark gaps are formed by short cylinders 19 extending through the equipotential plane 17 transversely thereto. Each end of each cylinder 19 is rounded and faces the rounded end of the adjacent cylinder 19 so as to form a spark gap. Between each pair of adjacent equipotential planes 17, the belt spacer supports 18 serve not only to provide increased mechanical strength for the equipotential plane 17 but also to support the belt spacers 20 and gradient bars 21 which alternate through the length of the column 1. Both the spacers 20 and the gradient bars 21 slide onto the belt spacer supports 18. The gradient bars 21 effectively present a fiat metallic surface toward the belt 13 but are sufiiciently recessed so that the belt 13 does not come in contact therewith. Each belt spacer assembly 20 comprises a series of glass or porcelain rods 22 which are cemented to the belt spacer strip 23 in axial alignment. The belt spacers 20 serve to insulate the charge on the belt 13 from the gradient bars 21, equipotential planes 17, and other metallic parts of the column structure.

An equipotential ring 14 is supported on each column plane assembly 15 by means of end bars 24, 25. The lower bar 24 is longer than-the upper bar 25 and supports an equipotential ring 14 at the extremities of said bar 24, while the shorter upper bar 25 provides a third point of support by means of a ring support saddle 26 afiixed to a ring support cup 27. Between the end bars 24, 25 near their extremities are supported belt spacers 28 and gradient bars 29 which are similar to those described above. These belt spacers 28 and gradient bars 29 seiye the external sides of the belt runs. The belt 12 is thus mechanically confined and electrically shielded throughout its length in travelling around the column 1. In order to provide an even distribution of potential along the column 1, between the high voltage terminal 9 and ground, a series of resistor assemblies 30 are supported between the short end bar 25 of each column plane assembly 15.

and the long end bar 24 of the adjacent column plane assembly 15. The stresses through the length of the column 1 are generally reduced by a heavy spring plane 31 which is held between the column-support clevis blocks 3 and column termination pads 32.

The advantages of the invention when used in connec tion with a horizontally mounted electrostatic generator may perhaps be better understood with reference to Figs. 6 through 12. Figs. 6, 8 and l0 show several construe tions of the column of an electrostatic generator mounted in cantilever. In the diagram of Fig. 6, a single beam 33 is firmly fixed to the base plate 34, as being cemented thereto, so that rotation is not permitted at the base. The resultant distribution of bending moments is shown in the moment diagram of Fig. 7. The bending moment increases exponentially from the high voltage terminal 35 to the base plate 34 with maximum bending moment at the junction between the column 33 and the base plate 34. That the increase should be faster than linear is evident from that fact that, as the point at which the bending moment is measured is moved toward the base plate 34, not only does the weight of the portion of the column 33 supported at said point increase lineally, but the distance from said point of the center of mass of the column weight being thus supported also increases lineally. Of course, the efi'ect'of the Weight of the high-voltage terminal 35 is also included in the moment diagram of Fig. 7.

In the columns shown in Figs. 8 and 10, the total bending moment at any point may be regarded as the sum of two components. One component is due to the weight of the column and the high-voltage terminal and its distribution is similar to that shown in the moment diagram of Fig. 7. This component is shown by the dotted line in the moment diagrams of Figs. 9 and 11. The other component is in the reverse direction and is introduced by the equipotential planes.

Referring now to Fig. 8, the column therein shown has the same amount of material as that shown in Fig. 6, but comprises two beams 36, 37 supported on the baseplate 34 in vertical alignment by two pins 38, 39, respectively, and a series of equipotentialplanes 40 equal in all respects are introduced as shown. .The highvoltage terminal 35 is supported on the two beams 36, 37 by two pins 41, 42, respectively. As shown in the moment diagram of Fig. 9, the bending moment component introduced by the equipotential planes 40 (shown by the discontinuous stepped line) increases in a more. or less linear fashion from the terminal end to the grounded end, thereby producing the resultant distribution of bending moments shown by the unbroken line. By stiffening the equipotential planes 40, the stepped line is rotated counter-clockwise about its right-hand extremity, thereby resulting in a lowering of the unbroken line. The effect of the equipotential planes 40, therefore, is to reduce the clockwise bending moments by an amount which increases with increased stiffness of the equipotential planes 40. However, as the clockwise bending moments are reduced, the counterclockwise bending moments are increased, sorthat there will be an optimum stiffness for the equipotential planes 40 for which the maximum bending moment in the column is minimized. In a preferred form of the invention, the bending moment at the grounded end of the column is reduced to zero and the column can be supported on a pair of pins 38, 39. In the column shown in Fig. 8 the high-voltage terminal 35 is displaced vertically until the equipotential planes 40 are sufficiently deformed to provide the required reverse bending moment. In order to avoid excessive displacement of the high-voltage terminal 35, the equipotential planes 40 should be stiffened as hereinbefore described. However, such an arrangement may result in an excessive reverse bending moment in the center of the column. To avoid this there is provided, in accordance with the invention, a heavy spring plane 43 near the base of the column, as shown in Fig. 10.. The bending-moment components and the resultant bending moments then take the distribution shown in the moment diagram of Fig. 11. It might be thought that the optimum distribution would be that for which the peak clockwise bending moment near the base of the column is equal to peak the counterclockwise bending moment in the center of the column. However, the total stresses in the column include not only those associated with the bending moments but also the direct tensional and compressional stresses, which rise to a peak at the base of the column in an exponential fashion. In order to minimize the maximum combined stresses in the column, the bending moment at the base of the column should be less than that at the center of the column. The foregoing is illustrated graphically in the stress diagram of Fig. 12.

The principal mechanical effect of adding the belt spacer supports 18 is to increase the moment of inertia of the horizontal cross-section of the equipotential planes 17 about its longitudinal central horizontal axis. Thus, referring to Fig. 5, the moment of inertia I of the section therein shown is the sum of that of the equipotential plane 17 plus those of the belt spacer supports 18, so that, assuming the section of the belt spacer supports 18 to be equivalent to the rectangle shown by the broken lines,

I= ab +%cd 1 The displacement of each equipotential plane 17 is inversely proportional, not only to I, but also to the modulus of elasticity E of the material in question. Since the belt spacer supports 18 are made of aluminum while the equipotential plane 17 is made of stainless steel, the gain in stiffness obtained by adding the belt spacer supports 18 is not so great as that indicated by Equation 1. However, the light weight of aluminum makes it preferable to stiffer but heavier materials.

Having thus described the principles of the invention together with a preferred embodiment thereof, it is to be understood that although specific terms are employed they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims. In particular, while the invention has been described herein with particular reference to a horizontally mounted electrostatic generator and while a column constituted in accordance with the invention has special advantages when horizontally mounted, it should again be emphasized that the invention is not limited thereto but also includes vertically mounted columns. While the problems of cantilever support are avoided in a vertically mounted electrostatic generator, the greatly increased compactness aiforded by the column construction of the invention renders a column so constructed advantageous in vertically mounted generators as well as generators horizontally mounted.

I claim:

1. An electrostatic generator comprising in combination a hollow electrode, an endless belt for conveying electric charge to said hollow electrode, at least two fixed pins horizontally disposed and supported in vertical alignment, a column horizontally mounted in cantilever upon said two fixed pins and adapted to support and insulate said hollow electrode, said column com- 6 prising a multiplicity of equipotential planes separated by insulators arranged in at least two rows in vertical alignment, and means to control the voltage of said equipotential planes, whereby said equipotential planes serve not only to control voltage gradients in said column but also to redistribute the bending moments in the insulators and in the seals between the insulators and the equipotential planes so as to reduce the maximum bending moment therein.

2. Apparatus in accordance with claim 1 wherein a heavy spring plane is provided at the base of the column.

3. An electrostatic generator comprising in combination a hollow electrode, an endless 'belt for conveying electric charge to said hollow electrode, at least two fixed pins horizontally disposed and supported in vertical alignment, a column horizontally mounted in cantilever upon said two fiXed pins and adapted to support and insulate said hollow electrode, said column having at least two pins horizontally disposed and supported on said column in vertical alignment and adapted to support said hollow electrode, said column comprising a multiplicity of equipotential planes separated by insulators arranged in at least two rows in vertical alignment, and means to control the voltage of said equipotential planes, said equipotential planes having sufficient stillness to introduce bending-moment components into the insulators and into the seals between the insulators and the equipotential planes, which bending-moment components oppose the bending moment components due to the weight of the column and the hollow electrode in such a manner as to redistribute the net bending-moments so as to reduce the maximum bending-moment.

4. Apparatus in accordance with claim 3 wherein the stiffness of at least one of the equipotential planes near the grounded end of the column is markedly greater than the stiffness of the other equipotential planes.

5. Apparatus in accordance with claim 3 wherein said equipotential planes are provided with bars which serve to increase the moment of inertia of the cross-section of each of said planes about its horizontal central axis, thus restraining flat plate bending of these planes.

6. Apparatus in accordance with claim 5 wherein said equipotential planes comprise stainless steel and said bars comprise aluminum whereby increased stiffness is obtained without increasing the weight of the column unduly.

7. Apparatus in accordance with claim 5 wherein said bars alternately provide the support for belt spacers and gradient bars, respectively.

8. Apparatus in accordance with claim 7 wherein the belt spacers are supported by spring clips readily slidable lengthwise on said bars.

9. An electrostatic generator comprising in combination a hollow electrode, a column of rectangular section adapted to support and insulate said hollow electrode and an endless belt for carrying charge to said hollow electrode, said belt traveling about said column upon pulleys supported beyond the extremities of said column.

10. Apparatus in accordance with claim 9 wherein the pulleys upon which said belt is supported comprise respectively an inverted motor and an inverted alternator.

No references cited. 

